Molybdenum based oxidation catalysts

ABSTRACT

This invention concerns catalysts comprising a molybdenum compound of formula I, II, III, IV or V
 
V q MoA y O z   I
 
NiMo x B y O z′   II
 
VNi w Mo x C y′ O z″   III
 
CoNi w Mo x D y O z′″   IV
 
VNi w Co r Mo x E y O z″″   V
 
wherein: A is at least one cation selected from the group consisting of cations of: Cr, Sb, Co, Ce and Pb; B is at least one cation selected from the group consisting of cations of: Sb, Al and W; C is at least one cation selected from the group consisting of cations of: Fe, Zn, Al, Sb, Bi, W, Li, Ba, Nb and Sn; D is at least one cation selected from the group consisting of cations of: Ba, Mn, Al, Sb, Sn, and W; E is at least one cation selected from the group consisting of cations of: Fe, Ca, Mn, Sr, Eu, La, Zr, Ga, Sn and Pb; q, r, w, x and y are each independently a number from 0.1 to 10 and y′ is a number from 0 to 10, z, z′, z″, z′″, and z″″ are determined using the amounts and oxidation states of all cations present in each formula. These catalysts can be used in C 4  oxidation processes, especially butane oxidation processes.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 09/878,645, filed Jun. 11, 2001 now U.S. Pat. No. 6,500,779 which is a divisional of application Ser. No. 09/269,211, filed Mar. 22, 1999, now U.S. Pat. No. 6,271,169 which is a national phase filing under 35 U.S.C. 371 of International Application Number PCT/US97/16563, filed 18 Sep. 1997, which claims priority benefit from Provisional Application No. 60/026,597, filed Sep. 24, 1996.

FIELD OF THE INVENTION

This invention relates to compounds comprising molybdenum, oxygen and certain cations, and the use of these compounds as catalysts in C₄ oxidation processes, especially butane oxidation processes.

TECHNICAL BACKGROUND

Oxidative organic processes are widely used in industrial operations. One commercially valuable process involves the oxidation of butane to maleic anhydride. Maleic anhydride is used as a raw material for products ranging from agricultural chemicals, paints, paper sizing and food additives to synthetic resins. To fill the high demand for this valuable chemical, a variety of commercial processes have been developed.

One important route to maleic anhydride involves the vapor phase oxidation of n-butane over a vanadium/phosphorus oxide (VPO) catalyst. The reaction step involves oxidation of n-butane with air (oxygen) to form maleic anhydride, carbon oxides, water and smaller amounts of partially oxidized by-products. Typically, the process is carried out in fixed-bed reactors, fluid-bed reactors, or more recently in recirculating solids reactors having two reaction zones in which two separate reactions take place with a catalyst (the solid) circulating between the two reaction zones and taking part in reactions in both zones.

A number of non-VPO catalysts have been reported in the literature. Zazhigalov, V. A. et al., in an article entitled “Oxidation of n-butane on Vanadium Molybdenum-Oxide Catalysts”, Inst. Fiz. Khim. im. Pisarzhevskogo, Kiev USSR Neftekhimiya (1977), 17 (2),268-73 describe the activity of V₂O₅—MoO₃ catalysts in butane oxidation as passing through a maximum at 25% MoO₃, and that a certain catalytic structure consisting of, V⁴⁺, V⁵⁺ and Mo⁶⁺ ions correspond to their preferred catalyst composition. These results obtained at 500-600° C. indicate low catalyst activity at normal operating temperatures.

Mazzochia, C. R. et al., in “Selective Oxidation of Butane in the Presence of NiO—MoO₃ catalysts”; An. Quim. Ser. A 79, no. 1 108-113(1983) disclose nickel molybdate catalysts prepared by coprecipitation that exhibit low hydrocarbon conversions. At 475° C., 19% conversion of n-butane was noted with low selectivities to maleic anhydride.

Umit Ozkan and G. L. Schrader, in “Synthesis, Characterization and catalytic behaviour of cobalt molybdates for 1-butene oxidation to maleic anhydride”, Applied Catalysis, 23 (1986) 327-338 disclose the use of cobalt molybdate for the oxidation of 1-butene.

In spite of the progress in catalyst and process development over the years, a need still remains for improved non-VPO catalysts useful in the oxidation of C4 hydrocarbons, particularly n-butane, to maleic anhydride and especially catalysts which are active at lower temperatures and have shorter contact timnes; and it is to that end that the present invention is directed.

SUMMARY OF THE INVENTION

The present invention provides a catalyst, comprising a molybdenum compound of formula I, II, III, IV or V: V_(q)MoA_(y)O_(z)  I NiMo_(x)B_(y)O_(z′)  II VNi_(w)Mo_(x)C_(y′)O_(z″)  III CoNi_(w)Mo_(x)D_(y)O_(z′″)  IV VNi_(w)Co_(r)Mo_(x)E_(y)O_(z″″)  V wherein:

-   -   q is a number from 0.1 to 10;     -   r is a number from 0.1 to 10;     -   w is a number from 0.1 to 10;     -   x is a number from 0.1 to 10;     -   y is a number from 0.1 to 10;     -   y′ is a number from 0 to 10,     -   A is at least one cation selected from the group consisting of         cations of: Cr, Sb, Co, Ce and Pb;     -   B is at least one cation selected from the group consisting of         cations of: Sb, Al and W;     -   C is at least one cation selected from the group consisting of         cations of: Fe, Zn, Al, Sb, Bi, W, Li, Ba, Nb and Sn;     -   D is at least one cation selected from the group consisting of         cations of: Ba, Mn, Al, Sb, Sn, and W;     -   E is at least one cation selected from the group consisting of         cations of: Fe, Ca, Mn, Sr, Eu, La, Zr, Ga, Sn and Pb; and     -   z, z′, z″, z′″, and z″″ are determined using the amounts and         oxidation states of all cations present in each formula         according to the following equations:         z = ((q    times  oxidation  state  of  V) + (1    times    oxidation  state  of    Mo) + (y    times    oxidation  state  of  A))  divided  by  2  (oxidation  state  of  oxygen);         z^(′) = ((1  times  oxidation    state  of  Ni) + (x  times  oxidation  state  of  Mo) + (y  times  oxidation    state  of    B))  divided  by  2  (oxidation    state  of  oxygen);         z^(″) = ((1  times  oxidation  state  of  V) + (w  times  the  oxidation  state  of  Ni) + (x  times  oxidation  state  of  Mo) + (y^(′)  times  oxidation  state  of  C))  divided  by  2(oxidation  state  of  oxygen);         z^(′′′) = ((1  times  oxidation  state  of  Co) + (w  times  the  oxidation  state  of  Ni) + (x  times  oxidation  state  of  Mo) + (y  times  oxidation  state  of  D))  divided  by  2(oxidation  state  of  oxygen); and  z^(′′′′) = ((1  times  oxidation  state  of  V) + (w  times  the  oxidation  state  of  Ni) + (r  times  oxidation  state  of  Co) + (x  times  oxidation  state  of  Mo)   + (y  times  oxidation  state  of  E))  divided  by  2(oxidation  state  of  oxygen).

The present invention also provides a process for the oxidation of a C4 hydrocarbon to maleic anhydride, comprising: contacting the C4 hydrocarbon with a source of oxygen in the presence of a catalytic amount of a molybdenum catalyst comprising a compound of formula I, II, III or V, as defined above, to yield maleic anhydride.

The present invention further provides a process for the oxidation of n-butane to maleic anhydride, comprising: contacting n-butane with a source of oxygen in the presence of a catalytic amount of a molybdenum catalyst comprising a compound of formula IV, as defined above, wherein the molybdenum catalyst is in a bulk state, to yield maleic anhydride.

The present invention also provides a process for the oxidation of n-butane to maleic anhydride, comprising: contacting n-butane with a source of oxygen in the presence of a catalytic amount of a catalyst comprising a molybdenum compound of formula VI or VII in a crystalline, active phase V₉Mo₆O₄₀  VI V₂MoO₈  VII to yield maleic anhydride.

The present invention also provides a process for the preparation of a molybdenum compound comprising a crystalline oxide of formula I, II, III, IV or V, as described above, comprising the steps of: contacting at least one compound having a cation of the molybdenum compound with at least one cation containing compound for each of the other cations of the molybdenum compound in a solution comprising water to form a resultant solution or colloid; freezing the resultant solution or colloid to form a frozen material, freeze drying the frozen material; and heating the dried frozen material to yield the molybdenum compound of formula I, II, III, IV, V, VI or VII.

DETAILED DESCRIPTION OF THE INVENTION

Multicomponent catalyst systems have been identified herein wherein the presence of molybdenum in combination with other particular cations show a significant beneficial effect on catalyst performances compared with many non-VPO catalysts reported in the literature for n-butane oxidation. These new multicomponent catalysts of the present invention comprise a molybdenum compound of formula I, II, III, IV or V: V_(q)MoA_(y)O_(z)  I NiMo_(x)B_(y)O_(z′)  II VNi_(w)Mo_(x)C_(y′)O_(z″)  III CoNi_(w)Mo_(x)D_(y)O_(z′″)  IV VNi_(w)CO_(r)Mo_(x)E_(y)O_(z″″)  V wherein: q is a number from 0.1 to 10; r is a number from 0.1 to 10; w is a number from 0.1 to 10; x is a number from 0.1 to 10; y is a number from 0.1 to 10; and y′ is a number from 0 to 10.

A is at least one cation selected from the group consisting of cations of: Cr, Sb, Co, Ce and Pb. A is preferably Sb, and a preferred compound of formula I is V₁Mo₁Sb₁O_(z). B is at least one cation selected from the group consisting of cations of: Sb, Al and W. B is preferably Sb, and a preferred compound of formula II is Ni₁Mo_(2.3)Sb₁O_(z′). C is at least one cation selected from the group consisting of cations of: Fe, Zn, Al, Sb, Bi, W, Li, Ba, Nb and Sn. C is preferably Bi, or Nb and Sn, and preferred compounds of formula III are V₁Mo_(2.3)Ni₁Bi₁O_(z″) and V₁Mo_(2.3)Ni₁Nb₁Sn₁O_(z″). D is at least one cation selected from the group consisting of cations of: Ba, Mn, Mo, Al, Sb, Sn, and W. D is preferably Sn or W, and preferred compounds of formula IV are Co_(0.5)Ni_(0.5)Mo₃Sn_(0.5)O_(z′″) and Mo₃Co_(0.5)Ni_(0.5)W₁O_(z′″). E is at least one cation selected from the group consisting of cations of: Fe, Ca, Mn, Sr, Eu, La, Zr, Ga, Sn and Pb. E is preferably Fe, Sr, Zr, Ga or Pb, and preferred compounds of formula V are V₁Mo_(2.3)Ni_(0.5)Co_(0.5)Fe₁O_(z″″), V₁Mo_(2.3)Co_(0.5)Ni_(0.5)Sr₁O_(z″″), V₁Mo_(2.3)Co_(0.5)Ni_(0.5)Zr₁O_(z″″), V₁Mo_(2.3)Co_(0.5)Ni_(0.5)Ga₁O_(z″″) and V₁Mo_(2.3)Co_(0.5)Ni_(0.5)Pb₁O_(z″″).

The ranges for z, z′, z″, z′″, and z″″ defining the subscript for oxygen in the formulae I-V, varies widely. The value of z, z′, z″, z′″, and z″″ is defined using the range of possible oxidation states of all of the cations found in the molybdenum compound as shown below.

For the molybdenum compounds of formula I, the highest oxidation states of the A cations are: V⁵⁺, Mo⁶⁺, Cr⁶⁺, Sb⁵⁺, Co³⁺, Ce⁴⁺ and Pb⁴⁺; and the lowest oxidation states for the A cations are: V³⁺, Mo⁴⁺, Cr²⁺, Sb³⁺, Co²⁺, Ce³⁺ and Pb²⁺. Therefore, for example, when the molybdenum compound of formula I is V₁₀Mo₁A₁₀O_(z), wherein A is Cr, the maximum value for z. z_(max), is: z_(max)=((10×5)+(1×6)+(10×6))÷by 2=116/2=58. The minimum value for z, z_(min), in formula I wherein A is Cr is: z_(min)=((10×3)+(1×4)+(10×2))÷by 2=54/2=27. When the molybdenum compound of formula I is V_(0.1)Mo₁A_(0.1)O_(z), wherein A is Pb. z_(max)=((0.1×5)+(1×6)+(0.1×4))÷by 2=6.9/2=3.45. The minimum value for z, z_(min), in formula I wherein A is Pb is: z_(min)=((0.1×3)+(1×4)+(0.1×2))÷by 2=4.5/2=2.25. Thus, for molybdenum compounds of formula I, z ranges from 2.25 to 58.

For molybdenum compounds of formula II, the highest oxidation states are: Ni³⁺, Mo⁶⁺, Sb⁵⁺, Al³⁺, and W⁶⁺ and the lowest oxidation states are: Ni²⁺, Mo²⁺, Sb³⁺, Al³⁺, and W²⁺, z′_(max) and z′_(min) can be calculated as shown above for z_(max) and z_(min) of formula I.

For molybdenum compounds of formula III, the highest oxidation states are: V⁵⁺, Ni³⁺, Mo⁶⁺, Fe³⁺, Zn²⁺, Al³⁺, Sb⁵⁺, Bi⁵⁺, W⁶+, Li¹⁺, Ba²⁺, Nb⁵⁺, and Sn⁴⁺, and the lowest oxidation states are: V²⁺, Ni²⁺, Mo²⁺, Fe²⁺, Zn²⁺, Al³⁺, Sb³⁺, Bi³⁺, W²⁺, Li¹⁺, Ba²⁺, Nb³⁺, and Sn²⁺, z″_(max) and z″_(min) can be calculated as shown above for z_(max) and z_(min) of formula I. Since there can be mixtures of the C cations, these must be factored into the ranges for z.

For molybdenum compounds of formula IV, the highest oxidation states are: Co³⁺, Ni³⁺, Mo⁶⁺, Ba²⁺, Mn⁷⁺, Al³⁺, Sb⁵⁺, Sn⁴⁺, and W⁶⁺, and the lowest oxidation states are: Co²⁺, Ni²⁺, Mo²⁺, Ba²⁺, Mn²⁺, Al³⁺, Sb³⁺, Sn²⁺, and W²⁺, z′″_(max) and z′″_(min) can be calculated as shown above for z_(max) and z_(min) of formula I.

For molydbenum compounds of formula V, the highest oxidation states are: V⁵⁺, Ni³⁺, Co³⁺, Mo⁶⁺, Fe³⁺, Ca²⁺, Mn⁷⁺, Sr²⁺, Eu³⁺, La³⁺, Zr⁴⁺, Ga³⁺, Sn⁴⁺ and Pb⁴⁺, and the lowest oxidation states are: V²⁺, Ni²⁺, Co²⁺, Mo²⁺, Fe²⁺, Ca²⁺, Mn²⁺, Sr²⁺, Eu²⁺, La³⁺, Zr⁴⁺, Ga³⁺, Sn²⁺ and Pb²⁻, z″″_(max) and z″″_(min) can be calculated as shown above for z_(max) and z_(min) of formula I.

The catalysts of the present invention can be either a particular structure (containing a certain ratio of cations) or a combination of structures and thus comprise a mixture of the crystalline oxides of the molybdenum compound of formula I, II, III, IV or V and may further comprise the amorphous phase of the compound.

The molybdenum compounds of the present invention can be prepared by various methods, for example, by freeze drying, alcohol reflux and gel techniques. Spray roasting, spray drying and coprecipitation could also be employed. Ceramic methods, i.e., solid state techniques could be used but are, in general, less preferred. Certain compounds of formulas I-V are better prepared by one method over another as appreciated by one of ordinary skill in the art.

The catalyst preparative process is usually conducted at normal atmospheric pressure, but elevated or reduced pressures can be employed. Agitation is not required, but is usually provided to facilitate a homogeneous mix and to facilitate heat transfer.

One process for the preparation of a catalyst comprising a molybdenum compound of formulas I, II, III, IV, V, VI or VII, as described above, comprises contacting at least one cation containing compound with at least one cation containing compound for each of the other cations of the final molybdenum compound in a solution comprising water to form a resultant solution or colloid; freezing the resultant solution or colloid to form a frozen material, freeze drying the frozen material; and heating the dried frozen material to yield the molybdenum compound of formula I, II, III, IV, V, VI or VII.

The compound containing a cation of the final molybdenum compound of formula I, II, III, IV, V, VI or VII can be a salt, oxide or the like. The cation containing compounds are contacted with each other upon their addition to the solution comprising water. There is at least one cation containing compound for each of the cations in the molybdenum compound of formula I, II, III, IV, V, VI or VII. Each cation can be contained in a separate cation containing compound or more than one cation can be contained in the same cation containing compound. There can also be more than one cation containing compound for each cation in the final molybdenum compound of formulas I, II, III, IV. V, VI or VII. The cation containing compounds can be ammonium or sodium salts of molybdic acid, vanadic acid or tungstic acid, in addition to compounds containing the A, B, C, D, E cation of choice or other cations present in the final molybdenum compound of formulas I, II, III, IV, V, VI or VII. For example, the oxides or acetylacetonates, chlorides or acetates of Li, Cr, Mn, Mo, Fe, Co, Ni, Ce, W, Zn, Sr, Nb, Eu, Ba, Zr, Al, Ca, Sn, Pb, Sb, Bi, La and Ga can be cation containing compounds. Representative examples of such salts or oxides containing cations of the final molybdenum compound of formula I, II, III, IV, V, VI or VII can be found in Tables 1-6 of the present invention. Normal commercially available reagents can be used for the cation containing compounds used in the preparation of the molybdenum compound. The highest purity products attainable need not be employed; however the purity of the reagents must be known in order to calculate the gross amount required. In addition the reagents should not be contaminated with any catalyst poison. The amount of reagent employed should be within plus or minus 5%, preferably within plus or minus 2% of the amount indicated by stoichiometry.

The cation containing compounds are dissolved in an appropriate solvent to form a solution or fine colloid. The formation of a solutions as opposed to a colloid, is generally preferred for most effective mixing.

In cases where NH₄VO₃ is the cation containing compound used, NH₄VO₃ salt can be dissolved in water prior to adding the other cation. containing compounds by heating water or other appropriate solvents. The additional cation containing compounds can then be added to form a resultant solution or a finely mixed colloid. The resultant solution or colloid is rapidly frozen at liquid nitrogen temperatures. Rapid freezing ensures the cation containing compounds will remain intimately mixed and will not segregate to any significant degree. The frozen material can then be transferred to a freeze drier, such as a Virtis Freeze Drier (Baltimore, Md.) equipped with a Unitop unit. The solution is kept frozen while water vapor is removed by evacuation. In order to prevent melting of the frozen material, the freeze drier can be maintained at a temperature ranging from about 0° C. to about −40° C., preferably between −40° C. to −20° C. with a vacuum of 2-10 millitorr. After at least 24 hours, preferably about 2-4 days, the dried sample can be calcined (heated) in air at a temperature ranging from about 250° C. to about 500° C., preferably about 400° C., for a time sufficient to decompose the cation containing compounds of formula I, II, III, IV, V, VI or VII to form metal oxide phases. This will require heating for a period of about 0.5 hours to about 24 hours.

Freeze drying methods can produce mixtures of solid cation containing compounds which are nearly as well mixed as their solution counterparts. The method is superior to slower solvent evaporation because homogeneity can be lost during the removal of solvent. In addition, in many cases because the cation containing compounds are well mixed, a lower calcination temperature can be used, and can allow for the synthesis of metal oxides with higher surface areas than typically observed by traditional high temperature ceramic syntheses. Choice of calcination temperature and protocols and atmosphere can also influence the types of phases synthesized.

Another process for the preparation of a catalyst comprising a molybdenum compound of formula I, II, III, IV, V, VI or VII, as described above, comprises the steps of: mixing a solution comprising at least one cation containing compound for each of the cations in the molybdenum compound of formula I, II, III, IV, V, VI, VI or VII and an alcohol to form a suspension; heating the suspension to reflux; and isolating the molybdenum compound of formula I, II, III, IV, V, VI or VII.

At least one cation containing compound for each of the cations in the molybdenum compound of formula I, II, III, IV, V, VI or VII are combined with an alcohol or combination of alcohols, such as ethyl alcohol, ethyl alcohol/benzyl alcohol mixtures, 1-propanol, 1-butanol, isobutyl alcohol, 2-methyl-2 propanol, 1-heptanol, neopentyl alcohol, phenol or the like, to form a suspension. The cation containing compounds can be those as described above for the freeze drying process. Mixing or agitation can be supplied via methods known in the art. The preparation can be carried out in a dry box to prevent any hydration. The resulting suspension is next heated to reflux under an inert atmosphere, such as nitrogen, for a time sufficient to ensure a proper solution. After the solution is formed, the solvent can be removed by freeze drying, rotary-evaporation, or a slow drying process after filtering, decanting or the like. The dried material can then be calcined at a temperature ranging from about 250° C. to about 500° C., preferably about 400° C., under air for a time sufficient to form the molybdenum compound of formula I, II, III, IV, V, VI or VII. This can take from about 0.5 hours to about 24 hours.

A further process for the preparation of a catalyst comprising a molybdenum compound of formula I, II, III, IV, V, VI or VII, as described above, comprises the steps of: contacting at least one cation containing compound for each cation of the molybdenum compound of formula I, II, III, IV, V, VI or VII with at least one cation containing compound for each of the other cations in the molybdenum compound of formula I, II, III, IV, V, VI or VII in a solution comprising water to form a resultant solution or colloid; stirring the resultant solution or colloid until gelation occurs; and drying the gel to yield the molybdenum compound of formula I, II, III, IV, V, VI or VII.

The cation containing compounds for this gel process can be those as described above for the freeze drying process. Following contact of the cation containing compounds with each other, the resultant solution or colloid is stirred until gelation occurs. In some cases an adjustment of pH is needed to induce gelation. Acid or base can be added depending on the nature of the other ingredients being used in the preparation of the molybdenum compound. Gelation can occur at room temperature at atmospheric pressure. The resulting gel can be dried and then calcined at a temperature ranging from about 250° C. to about 450° C., preferably 400° C., under air for a time sufficient to thoroughly dry.

Compounds of formula I, II, III, and V of the present invention are useful as catalysts in the oxidation of C4 hydrocarbons. The present invention provides a process for the oxidation of C4 hydrocarbons to maleic anhydride, comprising: contacting the C4 hydrocarbon with a source of oxygen in the presence of a catalytic amount of a catalyst comprising a molybdenum compound of formula I, II, III or V as defined above. The C4 hydrocarbon is selected from the group consisting of: n-butane, 1-butene, 2-butene and butadiene, and isomers thereof.

In some instances the catalyst may be useful as a lattice oxygen catalyst in the oxidation of C4 hydrocarbons with the ability to selectively oxidize the C4 hydrocarbon in the absence of gas phase oxygen and thus can be the only source of oxygen.

The catalyst comprising the molybdenum compound of formula I, II, III or V can be used alone, supported on a catalyst support or impregnated in a carrier material. Typical support carrier materials are well known to those skilled in the art as are methods of preparing supported or impregnated catalysts. Typical materials comprise silica, titania, zirconia, alumina, thoria, silicon carbide and carbon. The catalyst comprising the molybdenum compound described herein can be used as isolated, or in cases where size and shape of the catalyst is dictated by the requirements of the equipment employed in the subsequent use of the catalyst, the catalyst can be processed or fabricated into various size and shape particles before use by grounding, pelletizing, briquetting, tabulating, or shaping in other ways as required.

A first group of compounds, listed in Table 1 as Examples 1-5, are of formula I: V_(q)MoA_(y)O_(z), where q varies from 0.1 to 10, y varies from 0.1 to 10, A is at least one cation selected from the group consisting of cations of Cr, Sb, Co, Ce and Pb, and z is calculated as above. The performance of these molybdenum compounds as catalysts in the oxidation of n-butane is shown in Table 1.

A second group of compounds, listed in Table 2 as Examples 6-9, are of formula II: NiMo_(x)B_(y)O_(z′), where x varies from 0.1 to 10, y varies from 0.1 to 10, B is at least one cation selected from the group consisting of cations of Sb, Al and W and z′ is calculated as above. The performance of these molybdenum compounds as catalysts in the oxidation of n-butane is shown in Table 2.

A third group of compounds, listed in Table 3 as Examples 10-21, are of formula III: VNi_(w)Mo_(x)C_(y′)O_(z″), where w varies from 0.1 to 10, x varies from 0.1 to 10, y′ varies from 0 to 10. C is at least one cation selected from the group consisting of cations of Fe, Zn, Al, Sb. Bi, W, Li, Ba, Nb and Sn and z″ is calculated as above. The performance of these molybdenum compounds as catalysts in the oxidation of n-butane is shown in Table 3.

Another group of compounds, listed in Table 5 as Examples 27-35, are of formula V: VNi_(w)Co_(r)Mo_(x)E_(y)O_(z″″), where w varies from 0.1 to 10, r varies from 0.1 to 10, x varies from 0.1 to 10, y varies from 0.1 to 10, E is at least one cation selected from the group consisting of cations of Fe, Ca, Mn, Sr, Eu, La, Zr, Ga, Sn and Pb and z″″ is calculated as above. The performance of these molybdenum compounds as catalysts in the oxidation of n-butane is shown in Table 5.

The present invention further provides a process for the oxidation of n-butane to maleic anhydride comprising contacting n-butane with a source of oxygen in the presence of a catalytic amount of a catalyst comprising a molybdenum compound of formula IV, as defined above, in a bulk state. In some instances the catalysts comprising a molybdenum compound of formula IV of the present invention are used as catalysts in the bulk state, i.e., not on a support, but as pure compounds. The catalyst comprising the molybdenum compound of formula IV may be the only source of oxygen. The catalytic oxidation can be carried out in a fixed or fluidized bed reactor.

A group of compounds, listed in Table 4 as Examples 22-26, are of formula IV: CoNi_(w)Mo_(x)D_(y)O_(z′″), where w varies from 0.1 to 10, x varies from 0.1 to 10, y varies from 0.1 to 10, D is at least one cation selected from the group consisting of cations of Ba, Mn, Al, Sb, Sn and W, and z′″ is calculated as above. The performance of these molybdenum compounds as catalysts in the oxidation of n-butane is shown in Table 4.

The present invention also provides a process for the oxidation of n-butane to maleic anhydride, comprising: contacting n-butane with a source of oxygen in the presence of a catalytic amount of a catalyst comprising a molybdenum compound of formula VI or VII in a crystalline, active phase V₉Mo₆O₄₀  VI V₂MoO₈  VII to yield maleic anhydride. Prepared as herein described, the molybdenum compound of formula VI or VII is formed with little MoO₃ by-product. The performance of this molybdenum compound in a crystalline, active phase as a catalyst in oxidation of n-butane is shown in Example 36 of Table 6.

Comparative Examples 37-41, not of this invention, and the performance of these materials as catalysts in the oxidation of n-butane is listed in Table 7.

Prior to use in the microreactor, the catalysts described herein are typically formed into a convenient catalyst shape by pelletizing the catalyst at about 30,000 psi (2.07×10⁶ kPa) or less, to form small disks and crushing the pellet through sieves. For fixed bed reactor evaluations, typically a −40, +60 mesh is used (U.S. Sieve Series). Optionally, one could blend the resultant powder with 1-3% of a die lubricant and pellet binder, such as graphite or Sterotex®, a hydrogenated cottonseed oil, commercially available from Capital City Products Company, Columbus, Ohio, before tabletting. For fluidized bed reactor use, the preferred size range is 20 to 150 micrometers.

Glass and stainless steel are usually employed as the material of construction for the microreactor. This is not critical as long as materials that contaminate the product with catalyst poisons are not employed.

Although processes of the invention are embodied in the following laboratory scale examples, Applicant notes that the invention can be practiced on an industrial scale by making the necessary engineering and design modifications which are customary in the art.

Catalytic oxidation using a catalyst comprising a molybdenum compound of formula I, II, III, IV, V, VI or VII can be carried out in a fixed or fluidized bed reactor or recirculating solids reactor. These catalysts can be utilized advantageously with regard to conversion and selectivity in the wide variety of conventional techniques and reactor configurations employed to conduct the vapor phase oxidation of C4 hydrocarbons to maleic anhydride. For example, the conversion can be conducted in a fixed-bed reactor, whereby the catalyst particles are maintained in a fixed position and are contacted with C4 hydrocarbon and a source of oxygen, typically molecular oxygen, both in appropriate amounts, optionally in the presence of one or more inert diluent gases, at a temperature varying between 200° C. and about 450° C., preferably between about 300° C. and about 350° C. The greatest advantages of using the catalyst of this invention are realized when the conversion of C4 hydrocarbon to maleic anhydride is carried out in a recirculating solids reactor, such as that described in U.S. Pat. No. 4,668,802. This patent discloses an improved process for the selective vapor phase oxidation of n-butane to maleic anhydride over a vanadium/phosphorus/oxygen (VPO) catalyst, whereby the amount of oxygen in the feed gas to the VPO catalyst is limited to less than the stoichiometric amount required for the total amount of n-butane converted in the process. The reduced catalyst resulting from the oxidation is separated from the gaseous product stream and is reoxidized, optionally in a separate reaction zone, before being contacted with n-butane.

The catalysts of the present invention demonstrate good results in activity, conversion and selectivity. Tables 1-6 below show conversion and selectivity after 1 sec and after 3 sec for various catalysts of the present invention and compare these results with those of other catalysts in the art (Table 7).

EXAMPLES

The reagents for the following examples are commercially available as follows: NH₄VO₃, Cr₂O₃, (CH₃CO₂)₇Cr₃(OH)₂, Ce(SO₄)₂, Fe(NO₃)₃₋₉H₂O, NbCl₅, Co(NO₃)₂-6H₂O, (CH₃CO₂)₂Mn-4H₂O, V₂O₅ and La(NO₃)₃₋₅H₂O from Aldrich, Milwaukee, Wis.; NH₄VO₃, MoO₃, Ni(OOCCH₃)₂-4H₂O, NiCl₂-6H₂O, H₂WO4, LiNO₃ (anhydrous), SnCl₂, (NH₄)₁₀W₁₂O₄₁-5H₂O, Co(NO₃)₂₋₆H₂O, and SnCl₂ from Alfa, Ward Hill, Mass.; (NH₄)₆Mo₇O₂₄-4H₂O), Ni(NO₃)-6H₂O, Al(NO₃)₃-9H₂O, Bi(NO₃)₃-5H₂O, Ni(NO₃)₂-6H₂O, Co(NO₃)₂-6H₂O, Al(NO₃)₃-9H₂O, Ca(NO₃)₂-4H₂O, and Ni(NO₃)₂, from Baker, Phillipsburg, N.J.; Pb(NO₃)₂, NiCl₂-6H₂O, Pb(NO₃)₂, and H₃PO₄ from EM Sciences, Gibbstown, N.J.; (NH₄)₆Mo₇O₂₄-4H₂O) from Mallinchkrodt, Erie, Pa.; NH₄VO₃, Sb(OOCCH₃)₃, Co(OOCCH₃)₂, Zr(SO₄)₂-4H₂O and Ga(NO₃)₃ from J&M, Ward Hill, Mass.; colloidal (Al₂O₃) from Nyacol, Ashland, Mass.; Zn(NO₃)₂-6H₂O from Fisher, Fairlane, N.J.; and Ba(NO₃)₂ and EuCl₂ from AESAR, Ward Hill, Mass.

General Procedure for Freeze Drying

The component salts (indicated in the tables) were added to the indicated amount of water. In cases where NH₄VO₃ was used, the salt was dissolved in water prior to adding the other components by bringing the solvent to a boil. The additional salts were then added to form the solution or finely mixed colloid/slurry. The resultant solutions/colloids were rapidly frozen in glass dishes (3″ diameter) using liquid nitrogen. The frozen material was then transferred to a Virtis Freeze Drier (Baltimore, Md.) equipped with a Unitop unit. In order to prevent melting of the frozen solid, the Unitop chamber shelves were maintained between −40 to −20° C. with a vacuum of 2-10 millitorr. After at least 24 hours (usually 2-4 days), the dried sample was calcined (heated) in air to 400° C. for 5 hours to produce the final catalyst product. Prior to microreactor evaluations, the material was pelletized at 20,000 psi and crumbled and screened on −40, +60 mesh screens.

General Procedure for Alcohol Reflux

Specific Preparation of V₂MoO₈

Vanadium pentoxide. V₂O₅, 90.94 g 0.500 mol, and molybdic oxide. MoO₃, 71.97 g, 0.500 mol were combined in an round bottomed flask equipped with an agitator with 1034 ml of isobutyl alcohol and 95 ml of benzyl. The resulting suspension was heated to reflux under a nitrogen atmosphere for 16 hours. The resulting green suspension was filtered and the isolated product dried and then calcined at 400° C. under air for 5 hours.

General Procedure for Gel Method

Specific Preparation of NiMo_(2.3)AlO_(z′)

35 ml of water was added to the colloidal alumina (20% by weight). The solid oxide and nickel acetate was added. The mixture was stirred until gelation occurred. The gel was dried then calcined at 400° C. under air for 5 hours.

Micro-Reactor Evaluation of Butane Oxidation Catalysts

The catalysts were pelletized at 1.38×10⁶ kPa into disks and subsequently crushed and sieved through (−40, +60) mesh screens. Approximately 0.9 cc of catalyst were used for each evaluation.

The catalyst testing facility consisted of six micro-reactors which were connected to a common feed source and a common analytical gas chromatograph (GC). Each of the micro-reactors consisted of a 5.0 cm by 0.64 cm stainless steel tube which was immersed in an individual sandbath to maintain isothermal conditions. The feed composition and individual reactor flow rates were metered by commercially available mass flow controllers (Tylan Model FC-260, available from Tylan Corp., Torrance, Calif.). All exit gas lines were heated to 200° C. and connected to a multiport Valco valve for the on-line analysis of products using a commercially available GC (Hewlitt-Packard 5890 Series II, Hewlitt-Packard, Palo Alto, Calif.). A computer program controlled the Valco valve to select a reactor or feed stream to fill the 0.5 ml sample loop for injection in the GC. The GC was used to analyze for butane, maleic anhydride, acetic acid, acrylic acid, other C₁ to C₄ hydrocarbons, oxygen, carbon monoxide, carbon dioxide, nitrogen and water.

The standard testing protocol for butane oxidation catalysts was developed to measure maleic anhydride selectivities and yields under hydrocarbon-lean conditions (2% n-butane, 20% oxygen) over a range of butane conversions. Temperature was varied from 350° to 380° to 400° and back to 350° C., with three contact times (nominally 3, 1, and 0.5 s) evaluated at each temperature. The temperature was returned to 350° C. to provide information about the equilibration of the catalyst. The conversion and selectivity of two representative evaluations (at 380° C.) are reported in the tables below.

TABLE 1 380° C., 2.0% Butane/Air 1 sec 3 sec Composition Gram Mole Method of Conversion Selectivity Conversion Selectivity V_(q)MoA_(y)O_(z) Example Salt Used Weight Solvent Synthesis % % % % V₁Mo₁Cr₁O_(z) 1 NH₄VO₃, 50% 116.98 250 ml H₂O freeze-dried 41 6 75 2 V₂O₅ 50% 181.90 Cr₂O₃ 151.99 (CH₃CO₂)₇Cr₃(OH)₂ 602.32 (NH₄)₆Mo₇O₂₄—4H₂O 1235.90 MoO₃ 143.94 V₁Mo₁Sb₁O_(z) 2 NH₄VO₃ 116.98 250 ml H₂O freeze-dried 75 7 99 2 (NH₄)₆Mo₇O₂₄—4H₂O 1235.90 Sb(OOCCH₃)₃ 298.88 V₁Mo₁Co₁O_(z) 3 NH₄VO₃ 116.98 250 ml H₂O freeze-dried 70 3 99 0 (NH₄)₆Mo₇O₂₄—4H₂O 1235.90 Co(OOCCH₃)₂ 177.023 V₁Mo₁Ce₁O_(z) 4 NH₄VO₃ 116.965 250 ml H₂O freeze-dried 18 9 45 4 (NH₄)₆Mo₇O₂₄—4H₂O 1235.9 Ce(SO₄)₂ 332.24 V₁Mo₁Pb₁O_(z) 5 NH₄VO₃ 116.965 250 ml H₂O freeze-dried 15 8 33 4 (NH₄)₆MO₇O₂₄—4H₂O 1235.9 Pb(NO₃)₂ 331.2

TABLE 2 380° C., 2.0% Butane/Air Method 1 sec 3 sec Composition Gram Mole of Conversion Selectivity Conversion Selectivity NiMo_(x)B_(y)O_(z′) Example Salt Used Weight Solvent Synthesis % % % % Ni₁Mo_(2.3)Sb₁O_(z′) 6 Sb(OOCCH₃)₃ 298.88 59 cc of freeze- 10 21 26 12 (NH₄)₆Mo₇O₂₄—4H₂O 1235.9 30% HCl dried Ni(OOCCH₃)₂—4H₂O 248.82 Ni₁Mo_(2.3)Al₁O_(z′) 7 Ni(OOCCH₃)₂—4H₂O 248.8212 35 ml H₂O gel 21  6 46 <1 (NH₄)₆Mo₇O₂₄—4H₂O 1235.9 colloidal (Al₂O₃) 20% 101.9613 by wt. Ni₁Mo_(1.3)W₁O_(z′) 8 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 50 ml H₂O freeze- 10 13 24  2 NiCl₂—6H₂O 237.71 70 ml NH₄OH dried H₂WO₄ 249.86 Ni₁Mo₁W_(1.3)O_(z′) 9 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 40 ml H₂O freeze- 10 11 35  0 NiCl₂—6H₂O 237.71 90 ml NH₄OH dried H₂WO₄ 249.86 25 ml H₂O

TABLE 3 380° C., 2.0% Butane/Air Ex- Method 1 sec 3 sec Composition am- Gram Mole of Conversion Selectivity Conversion Selectivity VNi_(w)Mo_(x)C_(y′)O_(z″) ple Salt Used Weight Solvent Synthesis % % % % V₁Mo_(2.3)Ni₁O_(z″) 10 NH₄VO₃ 116.98 100 cc H₂O freeze- 40 21 71 15 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 100 cc H₂O dried NiCl₂—6H₂O 237.71 20 cc HCl V₁Mo_(2.3)Ni₁Fe₁O_(z″) 11 NH₄VO₃ 116.98 100 cc H₂O freeze- 28  7 60 <1 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 100 cc H₂O dried Ni(NO₃)—6H₂O 290.81 20 cc HNO₃ Fe(NO₃)₃—9H₂O 404 V₁Mo_(2.3)Ni₁Zn₁O_(z″) 12 NH₄VO₃ 116.98 100 cc H₂O freeze- 53  6 83 <1 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 100 cc H₂O dried Ni(NO₃)—6H₂O 290.81 20 cc HNO₃ Zn(NO₃)₂—6H₂O 297.47 V₁Mo_(2.3)Ni₁Al₁O_(z″) 13 NH₄VO₃ 116.98 100 cc H₂O freeze- 45  5 79  0 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 100 cc H₂O dried Ni(NO₃)—6H₂O 290.81 20 cc HNO₃ Al(NO₃)₃—9H₂O 375.13 V₁Mo_(2.3)Ni₁Sb₁O_(z″) 14 NH₄VO₃ 116.98 100 cc H₂O freeze- 39 10 69  5 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 100 cc H₂O dried Ni(NO₃)—6H₂O 290.81 15 cc HNO₃ Sb(OOCCH₃)₃ 298.88 50 cc HCl V₁Mo_(2.3)Ni₁Bi₁O_(z″) 15 NH₄VO₃ 116.98 100 cc H₂O freeze- 20 15 50  7 (NH₄)₆Mo₇O₂₄—H₂O 1235.86 100 cc H₂O dried Ni(NO₃)—6H₂O 290.81 20 cc HNO₃ Bi(NO₃)₃—5H₂O 485.07 30 cc HNO₃ V₁Mo_(2.3)Ni₁W₁O_(z″) 16 NH₄VO₃ 116.98 100 cc H₂O freeze- 30 10 71  2 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 100 cc H₂O dried Ni(NO₃)—6H₂O 290.81 20 cc HNO₃ H₂WO₄ 249.86 V₁Mo_(2.3)Ni₁Li₁O_(z″) 17 NH₄VO₃ 116.98 100 ml H₂O freeze- 11 17 20 11 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 dried NiCl₂—6H₂O 237.71 15 ml HCl LiNO₃ (anhydrous) 68.9459 V₁Mo_(2.3)Ni₁Ba₁O_(z″) 18 NH₄VO₃ 116.98 100 ml H₂O freeze- 35 10 66  2 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 100 ml H₂O dried Ni(NO₃)—6H₂O 290.81 20 ml HNO₃ Ba(NO₃)₂ 261.3398 V₁Mo_(2.3)Ni₁Nb₁Fe₁O_(z″) 19 NH₄VO₃ 116.98 100 cc H₂O freeze- 30  3 60 <1 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 50 cc H₂O dried NiCl₂—6H₂O 237.71 20 cc HCl NbCl₅ 270.71 70 cc HCl Fe(NO₃)₃—9H₂O 404 V₁Mo_(2.3)Ni₁Nb₁Sn₁O_(z″) 20 NH₄VO₃ 116.98 100 ml H₂O freeze- 40 12 68  3 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 70 ml H₂O dried NiCl₂—6H₂O 237.71 20 ml HCl NbCl₅ 270.17 20 ml HCl SnCl₂ 189.61 50 ml H₂O V₁Mo_(2.3)Ni₁W₁Sn₁O_(z″) 21 (NH₄)₁₀W₁₂O₄₁— 3132.64 140 ml freeze- 25 10 47  3 5H₂O 1235.86 NH₄OH dried (NH₄)₅Mo₇O₂₄—4H₂O 116.98 NH₄VO₃ 290.81 50 ml H₂O Ni(NO₃)₂—5H₂O 189.61 SnCl₂ 5 ml HCl

TABLE 4 380° C., 2.0% Butane/Air 1 sec 3 sec Composition Exam- Gram Mole Method of Conversion Selectivity Conversion Selectivity CoNi_(w)Mo_(x)D_(y)O_(z″) ple Salt Used Weight Solvent Synthesis % % % % Co_(0.5)Ni_(0.5)Mo₃ 22 Co(NO₃)₂—6H₂O 291.03 20 ml H₂O freeze- 12.5 32 29 27 Sn_(0.5)O_(z′″) SnCl₂ 189.61 dried (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 50 ml H₂O NiCl₂—6H₂O 237.71 20 ml HNO₃ Mo₃Co_(0.5)Ni_(0.5) 23 Co(NO₃)₂—H₂O 291.03 20 ml H₂O freeze- 1.3  4 2.3 11 Mn₁O_(z′″) (CH₃CO₂)₂Mn—4H₂O 245.09 dried (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 30 ml H₂O NiCl₂—6H₂O 237.71 20 ml H₂O Mo₃Co_(0.5)Ni_(0.5) 24 Co(NO₃)₂—6H₂O 291.03 20 ml H₂O freeze- 0.8 16 2.1 18 Ba₁O_(z′″) Ba(NO₃)₂ 261.34 dried (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 80 ml H₂O NiCl₂—6H₂O 237.71 20 ml H₂O Mo₃Co_(0.5)Ni_(0.5) 25 Co(NO₃)₂—6H₂O 291.03 20 ml H₂O freeze- 3.9 15 10.5  8 Al₁O_(z′″) Al(NO₃)₃—9H₂O 375.13 dried (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 150 ml H₂O NiCl₂—6H₂O 237.71 20 ml H₂O Mo₃Co_(0.5)Ni_(0.5) 26 Co(NO₃)₂—6H₂O 291.03 40 ml H₂O freeze- 4.2 27 14.2 15 W₁O_(z′″) (NH₄)₁₀W₁₂O₄₁—5H₂O 3132.64 dried (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 50 ml H₂O NiCl₂—6H₂O 237.71 10 ml H₂O

TABLE 5 380° C., 2.0% Butane/Air Ex- 1 sec 3 sec Composition am- Gram Mole Method of Conversion Selectivity Conversion Selectivity VNi_(w)Co_(r)Mo_(x)E_(y)O_(z′″) ple Salt Used Weight Solvent Synthesis % % % % V₁Mo_(2.3)Ni_(0.5)Co_(0.5) 27 NH₄VO₃ 116.98 50 ml H₂O freeze- 15 13 32 7 Fe₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 20 ml H₂O dried Co(OOCCH₃)₂ 177.02 20 ml HCl Ni(NO₃)₂—6H₂O 290.81 Fe(NO₃)₃—9H₂O 404 20 ml H₂O V₁Mo_(2.3)Ni_(0.5)Co_(0.5) 28 NH₄VO₃ 116.98 50 ml H₂O freeze- 14 10 34 4 Ca₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 dried Co(OOCCH₃)₂ 177.02 20 ml HCl Ni(NO₃)₂—6H₂O 290.81 5 ml HCl Ca(NO₃)₂—4H₂O 236.15 V₁Mo_(2.3)Co_(0.5)Ni_(0.5) 29 NH₄VO₃ 116.98 50 ml H₂O freeze- 12.5 2.5 29.2 1 Mn₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 20 ml H₂O dried Co(OOCCH₃)₂ 177.02 20 ml HCl Ni(NO₃)₂—6H₂O 290.81 (CH₃CO₂)₂Mn—4H₂O 245.09 10 ml H₂O V₁Mo_(2.3)Co_(0.5)Ni_(0.5) 30 NH₄VO₃ 116.98 50 ml H₂O freeze- 21 17 47 10  Sr₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 20 ml HCl dried Co(OOCCH₃)₂ 177.02 10 ml H₂O Ni(NO₃)₂—6H₂O 290.81 Sr(NO₃)₂ 211.63 V₁Mo_(2.3)Co_(0.5)Ni_(0.5) 31 NH₄VO₃ 116.98 50 ml H₂O freeze- 24 7.5 54 2 Eu₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 dried Co(OOCCH₃)₂ 177.02 20 ml HCl Ni(NO₃)₂—6H₂O 290.81 EuCl₂ 222.87 10 ml H₂O V₁Mo_(2.3)Co_(0.5)Ni_(0.5) 32 NH₄VO₃ 116.98 50 ml H₂O freeze- 17 10 36 5 La₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 dried Co(OOCCH₃)₂ 177.02 20 ml HCl Ni(NO₃)₂—6H₂O 290.81 La(NO₃)₃—5H₂O 415.01 10 ml H₂O V₁Mo_(2.3)Co_(0.5)Ni_(0.5) 33 NH₄VO₃ 116.98 50 ml H₂O freeze- 20 17.5 46 12  Zr₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 dried Co(OOCCH₃)₂ 177.02 20 ml HCl Ni(NO₃)₂—6H₂O 290.81 Zr(SO₄)₂—4H₂O 355.32 40 ml H₂O V₁Mo_(2.3)Co_(0.5)Ni_(0.5) 34 NH₄VO₃ 116.98 50 ml H₂O freeze- 18 16.5 43 11  Ga₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 dried Co(OOCCH₃)₂ 177.02 20 ml HCl Ni(NO₃)₂—6H₂O 290.81 Ga(NO₃)₃ 255.74 V₁Mo_(2.3)Co_(0.5)Ni_(0.5) 35 NH₄VO₃ 116.98 50 ml H₂O freeze- 22 20 47 14  Pb₁O_(z″″) (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 dried Co(OOCCH₃)₂ 177.02 20 ml HCl (with Ni(NO₃)₂—6H₂O 290.81 dry ice Pb(NO₃)₂ 331.2 100 ml HCl and 86 ml acetone) NH₄OH

TABLE 6 380° C., 2.0% Butane/Air 1 sec 3 sec Gram Mole Method of Conversion Selectivity Conversion Selectivity Composition Example Salt Used Weight Solvent Synthesis % % % % V₂Mo₁O_(x) 36 V₂O₅ 90.94 g 181.88 1034 ml isobutyl alcohol Alcohol 60 10 90 0 MoO₃ 71.97 g 143.94 95 ml benzyl alcohol reflux

TABLE 7 380° C., 2.0% Butane/Air 1 sec 3 sec Comp. Gram Mole Method of Conversion Selectivity Conversion Selectivity Composition Example Salt Used Weight Solvent Synthesis % % % % MoO₃ 37 (NH₄)₆Mo₇O₂₄—4H₂O 1235.9 freeze-dried 11 <1 VPO 38 NH₄VO₃ 116.965 freeze-dried 8 20 H₃PO₄ 97.97 Ni₁W_(2.3)O_(t) 39 H₂WO₄ 249.86 50 ml H₂O freeze-dried 2.5  0 9 0 NiCl₂—6H₂O 237.71 180 ml NH₄OH Ni₁Mo_(2.3)P₁O_(t) 40 Ni(NO₃)₂ 290.8 50 ml H₂O freeze-dried 6 12 (NH₄)₆Mo₇O₂₄—4H₂O 1235.86 100 ml H₂O H₃PO₄ 98 V₂O₅ 41 freeze-dried 80 <1 

1. A process for the oxidation of a C4 hydrocarbon to maleic anhydride, comprising: contacting a C4 hydrocarbon with a source of oxygen in the presence of a catalytic amount of a catalyst comprising a molybdenum compound of a formula selected from the group consisting of I, II, III, or V wherein: I≡V_(q)MOA_(y)O_(z) II≡NiMo_(x)B_(y)O_(z′) III≡VNi_(w)Mo_(x)C_(y)O_(2″) V≡VNi_(w)Co_(r)Mo_(x)E_(y)O_(z″″); wherein further: q is a number from 0.1 to 10; r is a number from 0.1 to 10; w is a number from 0.1 to 10; y is a number from 0.1 to 10; y′ is a number from 0.1 to 10; A is at least one cation selected from the group consisting of cations of Cr, Ce and Pb; B is at least one cation selected from the group consisting of cations of: Al and W; C is at least one cation selected from the group consisting of cations of: Fe, Zn, Al, Sb, Bi, W, Li, Ba, Nb and Sn; E is at least one cation selected from the group consisting of cation of: Fe, Ca, Mn, Sr, Eu, La, Zr, Ga, Sn and Pb; and z, z′, z″, and z″″ are determined using the amounts and oxidation states of all cations present in each formula according to the following equations: z = ((q    times  oxidation  state  of  V) + (1    times    oxidation  state  of    Mo) + (y    times    oxidation  state  of  A))  divided  by  2  (oxidation  state  of  oxygen); z^(′) = ((1  times  oxidation    state  of  Ni) + (x  times  oxidation  state  of  Mo) + (y  times  oxidation    state  of    B))  divided  by  2  (oxidation    state  of  oxygen); z^(″) = ((1  times  oxidation  state  of  V) + (w  times  the  oxidation  state  of  Ni) + (x  times  oxidation  state  of  Mo) + (y^(′)  times  oxidation  state  of  C))  divided  by  2(oxidation  state  of  oxygen); z^(′′′) = ((1  times  oxidation  state  of  Co) + (w  times  the  oxidation  state  of  Ni) + (x  times  oxidation  state  of  Mo) + (y^(′)  times  oxidation  state  of  D))  divided  by  2(oxidation  state  of  oxygen); and   z^(′′′′) = ((1  times  oxidation  state  of  V) + (w  times  the  oxidation  state  of  Ni) + (r  times  oxidation  state  of  Co) + (x  times  oxidation  state  of  Mo)   + (y^(′)  times  oxidation  state  of  E))  divided  by  2(oxidation  state  of  oxygen); to yield maleic anhydride.
 2. The process of claim 1 wherein the molybdenum compound is of formula III and C is Bi, or Nb and Sn.
 3. The process of claim 1 wherein the molybdenum compound is of formula V and E is Fe, Sr, Zr, Ga or Pb. 